Native or cellular prion protein, PrPC, is widely distributed throughout mammals and has a particularly well-conserved amino acid sequence and protein structure. Infectious prions, PrPSc, are composed of a modified form of the normal cellular prion protein. It is thought that a post-translational conformational change is involved in the conversion of non-infectious PrPC to infectious PrPSc during which α-helices are transformed into β-sheets. PrPC contains three α-helices and has little β-sheet structure, while PrPSc is abundant in β-sheet. Infectious prions are resistant to proteases, the enzymes in the body that can normally break down proteins.
The conversion of PrPC to PrPSc leads to development of transmissible spongiform encephalopathies (TSEs) during which PrPSc accumulates in the central nervous system and is accompanied by neuropathologic changes and neurological dysfunction. Infectious prions cause neurodegenerative disease by aggregating extracellularly within the central nervous system to form plaques known as amyloid that disrupt the normal tissue structure. This disruption is characterized by holes in the tissue with resultant spongy architecture due to vacuole formation in neurons. Other histological changes include astrogliosis and absence of an inflammatory reaction. While the incubation period for prion diseases is generally quite long, once symptoms appear the disease progresses rapidly, leading to brain damage and death. Neurodegenerative symptoms can include convulsions, dementia, ataxia (balance and coordination dysfunction), and behavioral or personality changes.
Specific examples of TSEs include scrapie, which affects sheep and goats; bovine spongiform encephalopathy (BSE); transmissible mink encephalopathy, feline spongiform encephalopathy and chronic wasting disease (CWD). In humans, TSE diseases may present themselves as kuru, Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler-Scheinker Syndrome (GSS), fatal insomnia, and variant Creutzfeldt-Jakob disease (vCJD).
No therapeutic agent exists to impede infectious prion propagation, and thus human TSE diseases are 100% fatal. A characteristic of all TSEs is the lack of a measurable host immune response to the agent. Consequently, no antibodies specific for TSEs have been identified. Moreover, the lack of a known nucleic acid sequence precludes the use of polymerase chain reaction based diagnostic methods. Thus, no conventional serologic test may be used to identify infected animals.
Methodologies that can readily separate or that can distinguish between two or more different conformational forms of a protein, e.g., PrPC and PrPSc, are needed to understand the process of conversion and to find structures that will specifically interact with the disease associated form. There also remains a need to identify high affinity binding materials that are specific for the conformationally altered protein and especially forms associated with disease.